I have a Sphere class that generate the VBO for creating given input radius and some more parameters.
Each Sphere VBO share the same memory layout (let's say vertex indice 0 = vertices, 1 = colors).
I may getting wrong, but if a understand correctly, state is what VAO store. But I don't know if VAO reminds which VBO was bound or use the currently bound VBO. But I think it use the VBO bound when modifying it (so it imply that reconfiguring the VAO for each Sphere render)
Question #1
Is it possible to store on VAO for all of the spheres? Sharing one VAO for multiple VBO.
Question #2
Can we set up the VAO independently from the VBO ? I wanna say, before even creating the VBO and without VBO bound, for example in a static function before even creating Spheres.
Question #3
This question may have non sens and be case-specific, but should I use one VAO and one VBO for each Sphere or share one VAO for all the Spheres (if it's possible) ?
Using the separate attribute format API, it is easy to set up a VAO independently from any particular set of source buffer objects. The vertex format information (component type, count, etc) can all be established via glVertexAttribFormat without a buffer object. You can even set up the relationship between attribute indices and buffer binding points, so that you can read interleaved attributes from a single buffer binding. All without ever binding a buffer object.
Now at the end of the data, you will have to attach buffer objects to the VAO when it comes time to render (via glBindVertexBuffer(s)). And the VAO will store the buffers it was last set with. But you should essentially ignore this. Treat a VAO as if it were just the vertex format state, with buffer binding being something that you do right before you render with it.
And yes, having one VAO and multiple potential buffers that get read with that vertex format (or better yet, just one buffer with multiple mesh data in it that all share the same vertex format, with mesh selection being done via the baseVertex parameter of glDrawElementsBaseVertex) is the right way to go.
All that being said, you also should remember that all spheres are the same mesh. They may appear in different locations with different sizes, but that's just a matter of providing a different transform of a unit sphere. The only reason to use more than one sphere mesh is if you need different resolutions of spheres (more polygons vs. fewer).